The present invention generally relates to the design of rotors for variable reluctance machines.
The switched reluctance machine (motor or generator) is a form of variable reluctance machine. The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, “The characteristics, design and application of switched reluctance motors and drives” by Stephenson and Blake, PCIM'93, Nürnberg, 21-24 Jun. 1993, incorporated herein by reference. A general treatment of the drives can also be found in various textbooks, e.g. “Electronic Control of Switched Reluctance Machines” by T J E Miller, Newnes, 2001. FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance machine 12 is connected to a load 19. The DC power supply 11 can be either a battery or rectified and filtered AC mains or some other form of energy storage. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the machine 12 by a power converter 13 under the control of the electronic control unit 14.
The switching must be correctly synchronised to the angle of rotation of the rotor for proper operation of the drive, and a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. Sensorless techniques for determining rotor position are also known to the person of ordinary skill in the art. Thus, the rotor position detector 15 may take many forms, including that of a software algorithm, and its output may also be used to generate a speed feedback signal. The presence of the position detector and the use of an excitation strategy which is completely dependent on the instantaneous position of the rotor leads to the generic description of “rotor position switched” for these machines.
One of the many known converter topologies is shown in FIG. 2, in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the “DC link” of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the “DC link capacitor”, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called “ripple current”) which cannot be drawn from or returned to the supply. In practical terms, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter. A resistor 28 is connected in series with the lower switch 22 to provide a non-isolated current feedback signal. An alternative current measurement arrangement 18 giving an isolated signal is shown in FIG. 1. A multiphase system typically uses several of the “phase legs” of FIG. 2 connected in parallel to energise the phases of the electrical machine.
The phase inductance cycle of a switched reluctance machine is the period of the variation of inductance for the, or each, phase, for example between maxima when the rotor poles and the relevant respective stator poles are fully aligned. FIG. 3 shows a typical inductance profile in relation to a corresponding motoring current waveform. As described in the Stephenson paper above, the winding is switched on to the supply at a rotor position θon and removed from the supply at θoff. The inductance is shown in idealised form, whereas in practice the corners of the profile are rounded due to flux fringing in the air and to saturation of the ferromagnetic paths.
Electrical machines in general are constructed from laminations of electrical sheet steel, the resulting structure being used to carry the magnetic flux on which the machine depends for its operation. The structure is laminated to reduce the effect of eddy currents, which flow in the steel due to the time rate of change of the flux. Usually only machines with unvarying flux have unlaminated structures. For example, the field structure of a dc machine can be unlaminated (i.e. solid), though even in these machines a laminated structure is often adopted in order to reduce the transient response for a new operating condition. The degree of lamination is usually decided by the frequency of flux variation in the machine. For example, in a machine energised directly from the 50 or 60 Hz mains supply and operating at, say, 1500 or 1800 rev/min, a lamination thickness of 0.50 or 0.65 mm is often adopted. For a machine operating on a 400 Hz supply and running at 10000 rev/min, a lamination thickness of 0.20 mm might be selected.
Of course, the decreasing lamination thickness brings many disadvantages, not least in terms of cost of material and of manufacturing difficulty, but the designer is usually prepared to accept these handicaps in order to gain the benefits of reduced eddy current loss, higher efficiency and higher specific output.
The output of the machine is also dependent on the so-called magnetising characteristic of the steel used. This is the relationship between the magnetising effort applied to the steel (i.e. the magneto motive force, mmf) and the flux consequently produced. While there is a range of grades of steel from which the designer can choose, all of them have the same general feature in that the initial, generally linear, relationship between mmf and flux gradually deteriorates with increasing mmf to the point of significant non-linearity. In practical terms, this represents a limit on the amount of flux that the steel can usefully carry—a state generally described as “saturated”, though this is not a particularly descriptive term, as there is no sharp cut-off in the relationship.
This magnetising characteristic of the steel becomes inseparably interrelated with the ideal characteristics of any machine in which the steel is used. For example, in a switched reluctance machine, the ideal inductance profile (discussed in the Stephenson paper above and shown in FIG. 3) takes on the steel characteristics and is significantly modified. This can be seen in the flux-linkage/angle/current relationships shown in FIG. 4, where the non-linearity of flux with current is immediately evident.
In general, the designers of electrical machines are under great pressure to design smaller and less costly machines to meet ever more demanding performance specifications. Size is important because it generally relates to both weight and cost, parameters which are important in the fields of, for example, aerospace and automobiles where fuel consumption is increasingly regarded as a major issue. At first sight, reducing the size for a given performance is simply achieved by working the steel harder, i.e. making it carry more flux in the given volume. Inspection of the curves of FIG. 4, however, shows that this results in a non-linear increase in the mmf, resulting in a non-linear increase in the ohmic losses of the machine and therefore in the thermal management problems associated with cooling. There is likely to be a consequential increase in the cost of the power converter for the machine. Thus there is clearly a practical limit that applies to the specific output of the machine.
A cross-section of a typical switched reluctance machine is shown in FIG. 5. The machine is doubly salient, i.e. both stator and rotor laminations have magnetically salient poles. In FIG. 5(a) the rotor is shown with a pair of poles fully aligned with the stator poles of Phase A. This represents the position of maximum inductance of the phase, often denoted Lmax, as shown in FIG. 3. In FIG. 5(b) the rotor has been rotated to the position where an inter-polar axis of the rotor is aligned with the stator poles. This represents the position of minimum inductance, denoted as Lmin. As the rotor rotates, the inductance varies between the extremes of Lmax and Lmin, giving the idealised form shown in FIG. 3. Typically, the rotor and the stator have the same axial length and the flux paths within them are notionally the same at any cross-section along that axial length. The axial lengths of the cores are often denoted as the ‘active length’ of the machine, the end-turns of the windings lying outside the active length at both ends of the machine.
A schematic flux path is shown in FIG. 5 and, while this considerably simplifies the complexity of the actual paths, it illustrates that the flux passes through the back-iron of the rotor as well as through the rotor poles, i.e. the back-iron region of the rotor is an integral part of the magnetic circuit associated with the phase winding. The flux path is essentially 2-dimensional. It will also be clear from FIG. 5(b) that the minimum inductance is heavily dependent on the length of the air path from the stator poles to the rotor back iron. FIG. 6 shows a sketch of the conventional shape of a two-pole rotor with the back iron section marked as region A.
In simple terms, the torque produced by a switched reluctance machine is proportional to the difference between Lmax and Lmin. The skill of the designer is brought to bear on the task of maximising this difference by increasing Lmax and reducing Lmin. However it will be seen that while reducing the rotor back-iron will tend to reduce Lmin, it will also reduce Lmax, so there is limited scope for improvement in this region.
U.S. Pat. No. 5,828,153 (McClelland), incorporated herein by reference, discloses a rotor of shaped lamination material in a particular type of switched reluctance machine with an external rotor.
Attempts to reduce Lmin by changing the direction of the flux path are generally hampered by the need to laminate the material to contain the losses. US Patent Application No 2004/0070301 (Mecrow), incorporated herein by reference, discloses an arrangement of rotor segments which have to be assembled on a shaft. These systems inevitably introduce mechanical complexity into the rotor design.
There is therefore an ongoing need for a cost-effective rotor design which reduces Lmin without significantly reducing Lmax.
Soft magnetic composite (SMC) material is a magnetisable material based on iron powder. It is generally pressed into the required finished shape rather than being punched and/or machined. Developments in powder metallurgy techniques have produced bonding agents which coat the iron powder and keep the resistivity high, so that the eddy currents in the material are reduced when the material is exposed to time varying flux. The material can be placed into a die and pressed to form the required component at pressures up to 800 MPa. The resulting components are then subjected to heat treatment at temperatures up to 500° C. A summary of material properties is given in “Soft Magnetic Composites—Materials and Applications”, Hultman & Jack, IEMDC Conference, Madison, Wis., USA, 1-4 Jun. 2003, Vol. 1, pp. 516-523, which is incorporated herein by reference.
In recent years, prototype designs have been proposed for permanent magnet, synchronous and reluctance machines which employ SMC material in some parts of the magnetic circuit. For example, “An Iron Composite Based Switched Reluctance Machine”, Alakula et al., Stockholm Power Tech, 18-22 Jun. 1995, Vol. 3, pp. 251-255, incorporated herein by reference, replaces the conventional laminations with SMC material, keeping the basic magnetic geometry unchanged. This project, however, failed to demonstrate any advantage over a conventional machine. A similar approach was taken in the work reported in “Design of a High Speed Switched Reluctance Generator for Aircraft Applications”, Duhayon et al, ICEM 2002, International Conference on Electrical Machines, 25-28 Aug. 2002, Brugge, Belgium, incorporated herein by reference. In general, it has been the intention of the designer to produce an item at least partly in SMC that is a mechanical, as well as functional, equivalent of a pre-existing rotor. No attempt has been made to exploit a soft magnetic material that can be formed without the limitations of laminations to address issue of increasing the difference between Lmax and Lmin.